It's good that you have a decent skimmer, that's helping, and you could be overfeeding, contributing to the issue, but I believe the root of your problem is the fact that you have 7 fish in a 29 gallon reef.... that's quite a bioload. On top of that, you're using a canister filter. Doesn't really matter how much you clean it... it's still converting 7 fish's worth of waste into nitrates.
Sounds like you have 2 options... get rid of the canister and a few fish, or be prepared to do larger pwc
's more often to prevent further coral loss.